All Optical Time Division Multiplexing - An alternative to WDM Professor Z. GHASSEMLOOY Optical Communications Research Group School of Engineering, Sheffield Hallam University UK Tel: +44 114 225 3274 Fax: +44 114 225 3433 email: [email protected]Wed add.: http//www.shu.ac.uk/ocr
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All Optical Time Division Multiplexing -An alternative to WDM
Professor Z. GHASSEMLOOY
Optical Communications Research GroupSchool of Engineering,
§ System approachú Theoretical Investigationú Simulation
• Analytical• Computer
ú Designú Implementation
All Optical Time Division Multiplexing an overview
All Optical Networks Layered Structure
Optical transmission layer
Optical multiplex layer
Optical channel layer
To overcome the bandwidth bottleneck due to opto-electronicor electro-optic conversion in existing network based on
optical transmission and electronic switching
Network Technologies
E/OMUX O/E DEMUX
Channels Channels
ElectricalBottlenecks
Fibre
E/O O/E
E/O
E/O
MUX
Channels
DEMUX O/E
O/E
Channels
Control signalOptical
Fibre
All Optical Multiplexing
Is the key in meeting the explosive bandwidth requirementof future communication networks!
§ Wavelength division multiplexing (WDM)
§ Optical time division multiplexing (OTDM) Π
§ Hybrid WDM-OTDM
E/O O/E
E/O
E/O
MUX
Channels
DEMUX O/E
O/E
Channels
OA
λ1 λ2… λM
WDM
§ Up to 90 wavelengths§ > 200 Gbps§ Transparent to data format and rate
Fibre
Problems with WDM
§ Nonlinearity associated with fibre, eg. Stimulated RamanScattering results in SNR degradation as the number ofchannel increases
§ Four wave mixing: limits the channel spacing§ Cross phase modulation: limits the number of channels§ High gain flat amplifiers§ Packet switched service by means of light paths: an
extremely inefficient way of utilizing network resources
Solution§ Optical Time Division Multiplexing (OTDM)
(introduced early 90’s)
OTDM – What does it offer?
§ Flexible bandwidth on demand at burst rates of 100 Gb/s perwavelength (in the longer term).
§ The total capacity of single-channel network = DWDM , but OTDM provide:ú potential improvements in network performance in terms of user
access time, delay and throughput, depending on the user rates andstatistics.
§ Less complex end node equipment (single-channel Vs. multi-channels)
§ Can operate at both:ú 1500 nm (like WDM) due to EDFAú 1300
§ Offers both broadcast and switched based networks
OTDM - History
§ 1968 T S Kinsel & R T Denton, IEEE Proc. 56,§ 1970 S J Buchsburn & R Kompfiner, ‘TDM Optical transmission systems’,
§ 1981 M Thewalt, - ‘OTDM using mode locked laser’, Elec. Lett.
§ 1985 S K Korothy et al - ‘ High speed LiNbO3 as switch/modulator forOTDM’
§ 1988 S Fujita - 10 Gbps systems
§ 1993 A D Ellis - 40 Gbps§ 1998 M Nakazawa - 640 Gbps (60 km)§ 1999 P Tolire, et al - 100 Gbps Packet based§ 2000 K Yanenasu – 80 Gbps (168 km zero dispersion fibre)
Progress in Raw Speed
1980 1985 1990 1995 2000 20050.1
1.0
100
1000
10
Bit
time
(ps )
Year
2.5 Gb/s
40 Gb/s
1.24Tb/sSub-picosecond region
Source: W H Knox, 2000
• 2000 - 40 Gb/s commercial product•1.28 Tb/s uses 200 fs pulses, using optical loop mirrors for demultiplexing (not possible electrically)
Ultrafast Technology Impact on WDM andOTDM
1 10 100 1000 100001.0
100
1000
10
Num
ber o
f wav
elen
gth
Data rate (Gb/s) /wavelength
1 Tb/s
100Tb/s
10 Tb/s
Ultrafast OTDM limits
Ultr
afas
t bro
adba
ndso
urce
• Source: DFB laser• 10000 WDN is achieved using spectral slicing
Ultrafast Pulse Source
10 100 10001
100
1000
10
50 G
Hz
Ban
dwid
th
WD
M c
hann
els
Pulse width (fs)
Commercial WDM 80 channels or more
OTDM Broadcastand
Switched Based Networks
A- Broadcast OTDM Networks
§ Bit interleaving:ú Each node will have a pair of OTDM Tx/Rx.ú Just like broadcast WDM networks, it requires multi-channel
media access protocol.- Star: Offers better link budget- Bus: Offers natural ordering of nodes, easier
synchronisation and protocol design.
§ Packet interleavingú No need for multi-channel media access controlú Requires single-channel media access protocol
1- Bit Interleaved - Multiplexer
Mode-lockedlaser
Splitter
Fibre delay iτ
Modulators
M1
MiCombiner
orstar coupler
OTDM
Data (NRZ)
Frame pulse
M(N-1)Fibre delay (N-1) τ
Fibre delay τ
Node i
Frame 1 Frame 2Time
Framing pulsesTip= Tb/N+1
Tb
• Data rate: 100 Mb/s: Ethernet at 10 Mb/s,Token Ring at 10 Mb/s,or FDDI at 100 Mb/s.
•Signal processing, switching and routing arecarried out during guard band time
Header Data
Packet OTDM - Multiplexer
Mode-locked
laserModulator 1st stage 2nd stage ith stage
Compression
OTDM
Mod. data
1 1 1 10 0
Compressedpacket
T-τ
2 (T-τ)
ab
c d e
a)
b)
c)
d)
e)
OTDM
Pulse i locationat the output is:(2k - 1)(T - t) +(i - 1)τ
Packet Compression
∑∑−
=
−
=
−δ==1
0
1
0
)()()(N
ii
N
iiin AiTttItI Ii(t) is the ith bit in the packet
Ai = 0 or 1
§ For a single bit the composed output is: ∑−
=+ τ−−=
1
01
)]([21
)(N
jini TjtItO
∑∑∑−
=
−
=+
−
=
τ++−==1
0
1
01
1
0
])([2
1 N
jji
N
in
N
iiout AjTjitIOI§ The output signal for
N bits packet is:
(T-τ) 2(T-τ) 4(T-τ) 8(T-τ)
Input Output
T τ
Iin(t) Iout
Packet OTDM- De-multiplexer
Splitter
Compressedpacket
&
&
&
&
&
Controlsignals
OTDM Demultiplexer
Ch- 1
Clock
DemuxOTDM
Clocksyn.
• Synchronisation of the control signal is essential to achieveaccurate demultiplexing. This requires the extraction of a clockcomponent from the received data using:
• electro-optical- PLL
• all optical at high speed > 40 Gbps- mode-locked fibre ring laser
Broadcast OTDM Networks - Problems
§ Large splitting loss§ No routing or switching (signals are sent to all nodes)
Solution:§ Switched based networksú Tune-ability: select any time slotú Routing and switchingú Much faster
§ Techniques used:ú Transmit at a much slower rate than packet itself,ú Allocate different wavelengthú Transmit as a separate sub-carrier channel on the same
wavelength
Switched Based OTDM Networks - Buffering
§ Buffer size depends on:ú Loadú Packet lossú No. of input/outputs in a S/W
- Practical: 23 or more limited:- by the number of possible re-circulations within a loop- by the excessive hardware for larger buffer depth.
Solution:- Multi-stage switched large optical buffer (>4000 for 8
input/output and 4 stages [D K Hunter et al, 1998]
Issues Associated with OTDM
OTDM - Issues
§ High speed electronics:ú Current ICs supporting 40 Gb/s will be available by 2001ú 40 Gb/s receiver are already available
§ Source: short pulse (tens of fs) and spectrally pure-• Gain switched semiconductor laser (broad source)• DFB laser (most suitable)• Mode locked fibre ring laser (pulse width < a few ps)• Semiconductor mode locked ( higher pulse width and not as pure as
MLFL)
§ Multiplexingú Electro-optically
§ 3 R regeneration:ú using semiconductor optical amplifier ∗ or nonlinear devices
∗ Passively
OTDM - Issues – cont.
§ Chromatics dispersion at 1550 nm is high, thus limiting the linkspan to 50 km at 200 Gb/s
• use dispersion shifted fibre b: But– can’t use the existing fibre– can’t compensate for a number of different wavelengths
• use dispersion equalisation techniques– by concatenating different fibres of opposite dispersion signs– chirped fibre Bragg gratings
• soliton pulse
§ Polarisation mode dispersion - limits the bit rate§ Demultiplexing techniques and devices Π§ Buffering Π
OTDM Demultiplexing Techniques
OTDM - Demultiplexing Schemes
Electro-optic
All optical
Optical Demultiplexers – All optical
§ Two main technologies:ú Kerr effect in optical fibresú Fast nonlinearities observed in semiconductor amplifiers
Types:
§ Optical loop mirror Π§ Interferometers with SA Π§ Four wave mixing
• CW through SLA will face phase shift• The SW window size = 2n∆x/c (~ a few ps)• Timing between control and data Pulses are critical• SLA recovery time is large ~300 500ps
All Optical Demultiplexing – Reported works
NOLM based§ 1994, S Kawanishi – 6.25 Gbps§ 1998, M Nakazawa - 640 Gbps§ 1999 , K S Lee – Terabit
TOAD based§ 1993, A D Ellis – 40 Gbps§ 1999 B Mekelson - 160 Gbps§ 1996, A J Poustie - All optical circulating shift register§ 1998, A J Poustie - Optical regenerative memory
OTDM- Demultiplexers
Performance Issues
Residual Crosstalk - NOLM
§ Depends on:ú control pulse rateú control pulse widthú walkoff time between control and signal pulses
cw control pulses ccw signal pulses
Coupler Loop mirror
Residual Crosstalk -TOAD
Depends on: - control pulse energy, - asymmetry, - SLA gain recovery time
Adjacent Channel Crosstalk
Depends on:• shape of the switching window• width of the signal pulse
§ Can be extended to an arbitary number of bits§ Only limited by the data and clock power available
§ SW window ∆t = τ
τ
2τ
4τ
∆t = τ
High Speed Optical Transport Layer[D M Spirit, et al, 1994]
OTDM Add/Drop Demultiplexer
ADM
Drop Add
Input Output
OTDM Cross Connect
ADM1
42 31 4321
AddDrop
ADM2
4321Add Drop
42 31
OTDM-λ1
OTDM-out
OTDM-λ2
LAN Application
Principle of 3R Regenerator
SDH
ATM
IP
SDH
ATM
IP
Open Optical Interface
SDH ATM IP Other
All Optical Networks
All Optical Networks / Existing Networks
OTDM and WDM - Comparison
§ 40 Gbps OTDM ≡ 16 Channels WDM 2.5 Gbps.§ For a 2 nm channel separation WDM would occupy the
whole of EDFA bandwidth. OTDM occupies only 1 nm ofwavelength space
§ OTDM does require an active demultiplexer and channelalignment systems. WDM may also require accuratecontrol of filter and source wavelength (DWDM).
§ OTDM uses the available optical spectrum moreefficiently.
§ OTDM is less well advanced, and more costly than WDM,BUT future research and progress may alter this.
Challenges Ahead
§ Packet Routingú Algorithmsú Bit error rate and Packet error rate Analysisú Dispersion control (use soliton)ú Crosstalk analysisú Bufferingú Intelligent based routing
§ OTDM Based Cross Connectsú Sizeú BER and Crosstalk analysis
Remarks
§ OTDM is a powerful technique for delivery of high capacity backbone,as an alternative (but not a substitute) to WDM
§ Nonlinear devices can be used as a an all optical demultiplexer
§ OTDM data rate can be increased and its performance improved byemploying soliton pulse
§ Commercial realisation depends on future advancements in integrationtechniques, and devices etc.
§ The development of the capacity is not the ONLY GOAL. The flexibleuse of this potential and advantages of optical routing are the KEYFACTORS in the development of optical network.
§ Technologies should develop, integrate with and enhance those alreadyexisting.
Acknowledgements
Professor M ADAMS (Surry University)Professor A K RAY (SHU)Professor P BALL (Fujitsu Telecomm. Research)Dr E. D. KALUARACHCHI - Lucent Technology, Bell Lab. US
Mr R. U. REYHER, - Network Designer, Siemens AGe, Germany
Dr U. SCHILLER - Researcher, Philips Semiconductors AG, Zurich
Dr R. WICKRAMASINGHE - Radio Network Planning Eng., Nokia Telecomm. Ltd,UKDr L CHAO - Associate Professor, Nanyang Technological University, Singapore
Dr L SEED - The University of Sheffield, UK
DR J M HOLDING, SHU, UK
Sheffield Hallam universityThe University of SheffieldMany undergraduate and postgraduate students